Stock Option Plan -Additional information (Detail) (USD $)	1 Months Ended	12 Months Ended				112 Months Ended	12 Months Ended				1 Months Ended								12 Months Ended			1 Months Ended				12 Months Ended	1 Months Ended					12 Months Ended								12 Months Ended
	Aug. 13, 2007	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2007	Dec. 31, 2012	Dec. 31, 2012 Employees	Dec. 31, 2012 Director	Dec. 31, 2012 Director Maximum	Dec. 31, 2012 Director Minimum	Dec. 31, 2012 Restricted Stock	Jul. 31, 2012 Restricted Stock	Dec. 31, 2011 Restricted Stock	Jan. 31, 2011 Restricted Stock	Dec. 31, 2010 Restricted Stock	Sep. 30, 2010 Restricted Stock	Jan. 31, 2010 Restricted Stock	Dec. 31, 2009 Restricted Stock	Dec. 31, 2012 Restricted Stock	Dec. 31, 2011 Restricted Stock	Dec. 31, 2010 Restricted Stock	Dec. 31, 2009 Restricted Stock Employees	Sep. 30, 2009 Restricted Stock Employees	Dec. 31, 2008 Restricted Stock Employees	Jan. 31, 2008 Restricted Stock Employees	Dec. 31, 2007 Restricted Stock Employees	Apr. 30, 2010 Restricted Stock Non Employee Directors	Mar. 31, 2010 Restricted Stock Non Employee Directors	Dec. 31, 2009 Restricted Stock Non Employee Directors	Sep. 30, 2009 Restricted Stock Board of Directors	Dec. 31, 2008 Restricted Stock Board of Directors	Dec. 31, 2012 Unvested Restricted Common Stock	Dec. 31, 2012 the "2003" plan	Dec. 31, 2012 the "2003" plan Amendment	Jun. 23, 2010 the "2003" plan Amendment	Jun. 04, 2009 the "2003" plan Amendment	Apr. 25, 2007 the "2003" plan Amendment	Apr. 26, 2006 the "2003" plan Amendment	May 31, 2012 the "2012 Plan"	Dec. 31, 2012 the "2012 Plan" Employees Maximum	Dec. 31, 2012 the "2012 Plan" Director Maximum	Dec. 31, 2012 the "2012 Plan" Consultants Maximum
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock reserved for future issuance																																	1,252,436	9,002,436	3,000,000	2,000,000	2,000,000	750,000	4,000,000
Outstanding options issued																																								6,284,408	862,645	250
Vesting period							3 years		3 years	2 years												3 years	1 year		3 years		1 year	1 year	2 years	1 year
Contractual terms							10 years	10 years
Additional shares reserved for issuance		45,823
Compensation expenses				$ 27,000	$ 120,000														$ 1,700,000	$ 635,000	$ 2,400,000
Options vested in period				6,904	25,111
Options exercised	25,111	3,683,786	2,911,186			3,683,786
Proceeds from stock options exercised	50,000	30,000	980,000	225,000		1,373,000
Total intrinsic value of options	104,000	11,000	2,500,000	880,000
Unrecognized compensation costs		$ 9,900,000				$ 9,900,000																										$ 2,800,000
Expected recognition period		1 year 8 months 23 days																														1 year 6 months 11 days
Restricted shares issued																						347,500	828,000	396,500	100,000	70,000	25,000	90,000	45,000	180,000	90,000
Repurchase of shares of restricted common stock											107,413	15,740	44,369	15,190	51,116	349,710	15,283	103,823
Repurchase of shares of restricted common stock, price per share											$ 4.19	$ 6.06	$ 4.41	$ 5.14	$ 4.66	$ 3.95	$ 3.10	$ 3.66	$ 4.19	$ 4.41	$ 4.66